A Vision Sensor, a Method of Vision Sensing, and a Depth Sensor Assembly

ABSTRACT

According to the present invention there is provided a vision sensor comprising, an array of pixels comprising rows and columns of pixels, wherein each pixel in the array comprises, a photosensor which is configured to output a current proportional to the intensity of light which is incident on the photosensor; a current source which is configured such that it can output a current which has a constant current level which is equal to the current level of the current output by the photosensor at a selected first instant in time, and can maintain that constant current level even if the level of the current output from the photosensor changes after said selected first instant in time; an integrator which is configured to integrate the difference between the level of current output by the current source and the level of current output by the photosensor, after the selected first instant in time; wherein the vision sensor further comprises a counter which can measure time, wherein the counter is configured such that it can begin to measure time at the selected first instant; and wherein each pixel in the array further comprises a storage means which can store the value on the counter at a second instant in time, the second instant in time being the instant when the integration of the difference between the level of current output by the current source and the level of current output by the photosensor of that pixel reaches a predefined threshold level. There is further provided a corresponding method of vision sensing, and a depth sensor assembly which comprises the vision sensor.

FIELD OF THE INVENTION

The present invention concerns a vision sensor comprising, a counter andan array of pixels, wherein each pixel in the array comprises, aphotosensor, a current source, an integrator and a storage means whichcan store the value on a counter when an output of the integratorreaches a predefined threshold. There is further provided a method ofvision sensing and a depth sensor assembly which uses the vision sensor.

BACKGROUND TO THE INVENTION

Three-dimensional vision is an important requirement for a variety ofinteresting machine vision applications; for example self-driving cars,autonomous robots, augmented reality devices, entertainment systems,gesture recognition, face tracking or 3D modelling.

Ranging devices such as lidars or time-of-flight cameras requiresub-nanosecond resolution to measure the time an emitted light pulsetravels to a surface and back. These kind of measurements demandexpensive setups either involving moving parts (lidar) or very complexand big pixel circuits (time-of-flight).

Passive vision systems, such as stereo vision or structure-from-motionovercome these limitations but require substantial computationalresources and are only functional in environments with sufficientlighting and spatial contrast.

Active vision systems, based on structured lighting on the other hand,combine the advantages of an active light source with the simple dataacquisition of a vision system.

In Active vision systems depth from structured lighting is obtained inthe following way: A well-known pattern is projected on to a scene. Thereflections of the pattern are captured by a camera which is mountedwith a fixed baseline distance to the projector. Geometrical constraints(epipolar geometry) and the captured position of a projected patternfeature allow inferring the depth of the underlying surface. In order toobtain dense depth maps, many small projected features are required. Toidentify these features they should either be unique such as in the caseof random dot patterns (e.g. Microsoft's Kinect) or multiplexed in time(e.g. Intel's Realsense or laser line scanners). Howeverdisadvantageously, the pattern of unique features limit the spatialresolution and require computationally expensive matching algorithms,and time-multiplexed patterns are constrained by the temporal resolutionof the sensor and can suffer from motion artefacts if the temporalresolution of the sensor is not sufficiently large compared to themotion captured in the scene.

It is an aim of the present invention to mitigate at least some of theabove-mentioned disadvantages.

BRIEF SUMMARY OF THE INVENTION

According to the invention, these aims are achieved by means of a visionsensor, comprising,

an array of pixels comprising rows and columns of pixels, wherein eachpixel in the array comprises, a photosensor which is configured tooutput a current proportional to the intensity of light which isincident on the photosensor; a current source which is configured suchthat it can output a current which has a constant current level which isequal to the current level of the current output by the photosensor at aselected first instant in time, and can maintain that constant currentlevel even if the level of the current output from the photosensorchanges after said selected first instant in time; an integrator whichis configured to integrate the difference between the level of currentoutput by the current source and the level of current output by thephotosensor, after the selected first instant in time;

wherein the vision sensor further comprises a counter which can measuretime, wherein the counter is configured such that it can begin measuretime at the selected first instant; and

wherein each pixel in the array further comprises a storage means whichcan store the value on the counter (i.e. the time measure on thecounter, referred to hear after as the ‘counter value’) at a secondinstant in time, the second instant in time being the instant when theintegration of the difference between the level of current output by thecurrent source and the level of current output by the photosensor ofthat pixel reaches a predefined threshold level.

Advantageously the vision sensor of the present invention is optimizedfor time-multiplexed structured lighting depth estimation that allows tominimize motion artefacts and to reduce the computational complexity.

Advantageously the vision sensor of the present invention achieves ahigh signal-to-noise ratio by in-pixel background subtraction using aconfigurable current source and precise temporal resolution by in-pixelillumination change detection.

A vision sensor may further comprise a processor which is configuredsuch that it can receive the counter value from each pixel in the array,and is configured to use the received counter value to generate a depthmap.

The storage means of pixels in the same column may be connected suchthat counter values stored in the storage means of a pixel may be passedto the storage means of the adjacent pixel.

The storage means may comprise a shift register. The shift register maycomprise one or more flip-flops.

The vision sensor may further comprise a read-out circuit for outputtingthe counter values stored in the storage means of one or more pixels inthe array to a processor which is configured to use the counter valuesto generate a depth map.

The read-out circuit may comprise a bus which can be sequentiallyconnected to the storage means of each pixel in the array and isconnected to the processor, so that counter values stored in the storagemeans of the pixels can be sequentially output to the processor acrossthe bus.

In one embodiment the storage means of pixels in the same column in thearray are connected such that counter values stored in the storage meansof a pixel may be passed to the storage means of the adjacent pixel, and

wherein the read-out circuit comprises a plurality of column circuitseach of which is configured to receive a counter value from the storagemeans of a pixel and to output the received counter value to theprocessor,

wherein a column circuit is provided for each column in the array ofpixels such that the number of column circuits correspond to the numberof columns in the array of pixels, and

wherein each column circuit is directly connected to a single storagemeans of a single pixel in a corresponding column, and wherein thecounter values stored in the storage means of other pixels in saidcolumn can be passed consecutively to the column circuit via the storagemeans of the pixel to which that column circuit is directly connected.

The read-out circuit may comprise a plurality of column circuits each ofwhich is configured to receive a counter value from the storage means ofpixels in a corresponding column, and to output the received countervalue to a processor,

wherein a column circuit is provided for each column in the array ofpixels such that the number of column circuits correspond to the numberof columns in the array of pixels, and

wherein for each pixel in each column a switch is provided between thestorage means of the pixel and the corresponding column circuit for thatcolumn, such that each column circuit can be selectively, directly,connected to the storage means of any of the pixel in a correspondingcolumn by closing the switch for that pixel; and

wherein the read-out circuit further comprises a row-select circuitwhich can select a row of pixels whose stored counter values are to beoutput to the processor, by selectively closing the switches for pixelsalong a row of the array, such that the storage means of each pixelalong said row is directly connected to respective corresponding columncircuits, so that each corresponding column circuit can simultaneouslyreceive the counter values stored in the storage means of the pixelslocated in a selected row, and subsequently output the received countervalues to the processor.

The column circuit may comprise a shift register which can receivecounter values stored in the storage means of the pixels in the array,and can sequentially output the received counter values to a processorconfigured to use the counter values to generate a depth map.

The photosensor may comprise a photodiode or phototransistor.

The photosensor may comprise a photodiode and a cascode NMOS transistor,wherein an output of the photodiode is connected to the source of theNMOS transistor, and the drain of the NMOS transistor defines the outputof the photosensor.

The photosensor may further comprise an inverting amplifier, wherein thegate of the NMOS transistor is connected to an output of the invertingamplifier and an input of the inverting amplifier is connected to anoutput of the photodiode.

The integrator may comprise a capacitor which can integrate thedifference between the level of current output by the current source andthe level of current output by the photosensor, after the selected firstinstant in time.

The integrator may comprise a comparator which determines if thedifference between the level of current output by the current source andthe level of current output by the photosensor has crossed saidpredefined threshold.

The comparator may be implemented using an amplifier.

The integrator may comprise a capacitor and wherein, an output of thecurrent source and an output of the photosensor are connected to a node,and the capacitor is connected between said node and ground, and whereinthe node is further connected to a first input of the amplifier, andwherein the a voltage source, which can define said threshold level, isconnected to a second input of the amplifier.

In an embodiment said capacitor is defined by parasitic capacitances ofthe current source, photosensor and amplifier at said node.

The vision sensor may further comprise a second amplifier, wherein thesecond amplifier is located between current source and the comparator.The second amplifier may be a capacitive amplifier comprising aninverting amplifier and two capacitors.

The current source of each pixel in the array may comprise a PMOStransistor.

In an embodiment the gate of the PMOS transistor is selectivelyconnected to an output of the amplifier by means of a switch which islocated between the gate of the PMOS transistor and an output of theamplifier, wherein the switch is closed at the first instant in time tocause the current source to output a current which has a constantcurrent level which is equal to the current level of the current outputby the photosensor, and wherein the switch is open between said firstinstant in time and second instant in time.

In an embodiment the current source further comprises a cascode PMOStransistor, wherein the drain the PMOS transistor is connected to thesource of the cascode PMOS transistor and the drain of the cascode PMOStransistor defines the output of the current source.

The vision sensor may further comprise a clock which is connected to thecounter so that the clock can provide a clock signal to the counter,wherein the clock signal defines the rate at which the counter counts.

The counter may be a binary counter which is configured to output abinary number which is representative of a counter value.

The counter may be a gray counter which if configured to output a graycode.

According to a further aspect of the present invention there is provideda method of vision sensing the method comprising the steps of, for oneor more pixels in an array of pixels,

(a) sampling background illumination the pixel at a first time instant;

(b) after the first time instant, integrating the difference between thesampled background illumination and a current illumination of saidpixel;

(c) measuring the time between the first time instant and a second timeinstant when the integral of the difference between the sampledbackground illumination and the current illumination of said pixelreaches a predefined threshold level.

The method may further comprise the step of, for each of said one ormore pixels, storing the counter value in a storage means of the pixel.

The method may further comprise the steps of, for each of said one ormore pixels, outputting the counter value to a processor; and at theprocessor, generating a depth map using the counter values.

The steps of outputting the counter values to a processor and generatinga depth map using the counter values, may comprise, outputting a binarynumber which is representative of the counter value, and generating thedepth map at the processor using said binary number.

The method may further comprise the step of, using the vision sensoraccording to any one of claims 1-26 to perform the steps (a)-(c).

The step (a) may comprise identifying the current which is output fromthe photosensor at the first time instant, and configuring the currentsource so that it outputs a current with a constant current level whichis equal to the current level of the current output by the photosensorat the first time instant, and maintaining that constant current leveluntil the second time instant at least, even if the level of the currentoutput from the photosensor changes after the first time instant.

The step (b) may comprise integrating the difference between the currentoutput from the photosensor and the current output by the currentsource.

The step of determining the difference between the sampled backgroundillumination and a current illumination of said pixel, may comprisedetermining the difference between the level of current output by thecurrent source and the level of current output by the photosensor.

In an embodiment of the method of vision sensing said current source ofeach pixel in the array comprises a PMOS transistor, and said integratorcomprises a comparator which determines if the difference between thelevel of current output by the current source and the level of currentoutput by the photosensor has crossed a predefined threshold; and anoutput of the current source and an output of the photosensor areconnected to a node, and the capacitor is connected between said nodeand ground, and wherein the node is further connected to a first inputof the comparator, and wherein the a voltage source, which can definesaid threshold level, is connected to a second input of the comparator;and wherein the gate of the PMOS transistor of the current source isselectively connected to an output of the comparator by means of aswitch which is located between the gate of the PMOS transistor and anoutput of the comparator; and wherein the method comprises the step ofdefining said threshold level by, before the first time instant, closingthe switch and providing a first voltage to the second input of thecomparator; at the first time instant, opening the switch and providinga second voltage to the second input of the comparator; wherein thedifference between the first voltage and second voltage defines thethreshold level.

In an embodiment of the method of vision sensing the storage means ofadjacent pixels in the same column in the array are connected, and themethod further comprises the step of passing the counter value which isstored in the storage means of a pixel to the storage means of anadjacent pixel located in the same column, so that the counter valuesstored in the storage means of pixels in the same column of the arraycan be outputted consecutively to a processor.

The method may further comprise the step of selecting a row of pixelswhose stored counter values are to be output to the processor, andsimultaneously outputting the counter values stored in the storage meansof all pixels in the selected row to a column circuit which isconfigured to output the received counter values to a processor.

According to a further aspect of the present invention there is provideda depth sensor assembly (200) comprising,

a projector (201); and

a camera (202) which comprises the vision sensor (1) according to anyone of the above-mentioned vision sensor embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood with the aid of the descriptionof an embodiment given by way of example and illustrated by the figures,in which:

FIG. 1a illustrates a vision sensor according to an embodiment of thepresent invention; FIG. 1b illustrates the features of each pixel thearray or pixels;

FIG. 2a illustrates a depth sensor assembly which uses the vision sensorof FIG. 1a ; FIG. 2b shows the resulting depth map which can be producedusing the depth sensor assembly of FIG. 2 a;

FIG. 3 illustrates the preferred configuration of a pixel used in thevision sensor;

FIG. 4 illustrates two exemplary configurations of photosensor that maybe used in each pixel of the vision sensor;

FIG. 5 illustrates two exemplary configurations of current sources thatmay be used in each pixel of the vision sensor;

FIG. 6 illustrates an exemplary configuration of an integrator that maybe used in each pixel of the vision sensor; the integrator shown in FIG.6 comprises of a capacitor and an amplifier;

FIG. 7 illustrates an exemplary configuration of an integrator that maybe used in each pixel of the vision sensor; the integrator shown in FIG.7 comprises a two-stage common source amplifier;

FIG. 8 illustrates an exemplary configuration of an integrator that maybe used in each pixel of the vision sensor; the integrator shown in FIG.8 comprises an 5-transistor operational transconductance amplifier;

FIG. 9 illustrates an exemplary configuration of an integrator that maybe used in each pixel of the vision sensor; the integrator shown in FIG.9 comprises a capacitive amplifier and a comparator;

FIG. 10a illustrates a vision sensor according to a further embodimentof the present invention; FIG. 10b illustrates the features of eachpixel the array or pixels, each pixel using an analogue storage means;

FIG. 11a illustrates a vision sensor according to a further embodimentof the present invention where the vision sensor comprises a readoutcircuit; FIG. 11b illustrates the features of each pixel the array orpixels;

FIG. 12a illustrates a vision sensor according to a further embodimentof the present invention where the storage means of the pixels may beconnected between adjacent pixels; FIG. 12b illustrates the features ofeach pixel the array or pixels;

FIG. 13a illustrates a vision sensor according to a further embodimentof the present invention where the vision sensor comprises a readoutcircuit and a row selection circuit; FIG. 13b illustrates the featuresof each pixel the array or pixels;

FIG. 14 illustrates an exemplary configuration of a readout circuit;

FIG. 15a illustrates a vision sensor according to a further embodimentof the present; FIG. 15b illustrates the features of each pixel thearray or pixels, where each pixel comprises two memories;

FIG. 16a illustrates a vision sensor according to a further embodimentof the present; FIG. 16b illustrates the features of each pixel thearray or pixels, where each pixel comprises multiple memories.

DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS OF THE INVENTION

FIG. 1a illustrates a vision sensor 1 according to an embodiment of thepresent invention.

The vision sensor 1 comprises an array of pixels 100 comprising rows andcolumns of pixels 101.

FIG. 1b illustrates the features of each pixel 101 the array or pixels.As shown in FIG. 1b , each pixel in the array comprises,

a photosensor 102 which is configured to output, at an output 102 a ofthe photosensor 102 a, a current proportional to the intensity of lightwhich is incident on the photosensor;

a current source 103 which is configured such that it can output, at anoutput 103 a of the current source, a current which has a constantcurrent level which is equal to the current level of the current outputby the photosensor at a selected first instant in time, and can maintainthat constant current level even if the level of the current output fromthe photosensor changes after said selected first instant in time;

an integrator 104 which is configured to integrate the differencebetween the level of current output by the current source and the levelof current output by the photosensor, after the selected first instantin time.

Referring back to FIG. 1a , the vision sensor further comprises acounter 108. The counter is configured such that it can begin to count(at a predefined rate), at a selected first instant in time. In thisembodiment each pixel in the pixel array is connected to the countersuch that each pixel can read the value on the counter. Preferably, thecounter is a binary counter. In the preferred embodiment the counter isa gray counter. The output of a gray counter is a gray code, which is abinary code where two successive values differ in only one bit.

Referring again to FIG. 1b , each pixel in the array further comprises astorage means 107; in this embodiment the storage means in each pixel isa memory 107. Each pixel can store the value of the counter 108 in itsrespective memory 107 at a second time instant, the second time instantbeing a time instant when the integration of the difference between thelevel of current output by the current source and the level of currentoutput by the photosensor of that pixel reaches a predefined thresholdlevel.

The integrator 104 has an input 104 a and an output 104 b. The output102 a of the photosensor 102 is connected to the input 104 a of theintegrator 104; the output 103 a of the current source 103 is alsoconnected to the input 104 a of the integrator 104. The output 104 b ofthe integrator 104 is connected to the memory 107.

During use, preferably at said selected first instant in time therespective current sources 103 of all of the pixels 101 in the pixelarray 100 are configured to output a current equal to the current outputby their respective photosensors 102; the respective current sources 103are configured to maintain an output at constant current equal to thecurrent output at the selected first instant in time. In each pixel 101the respective integrator 104 will integrate the difference between theconstant current output by the current source 103 and the current outputby the photosensor 102 (the current output of the photosensor willchange depending on the amount of light incident on the photosensor). Ata second instant in time, the integral of the difference between theconstant current output by the current source and the level of currentoutput by the photosensor of that pixel reaches a predefined thresholdlevel and the value on the counter 108 is stored in the memory/storagemeans 107 of that pixel; it should be understand that this may occur atdifferent times in each pixel in the pixel array, but the second instantin time may be the same or different for each pixel in the pixel array.At a selected third instant in time, the content of the memory/storagemeans 107 of a pixel may be read-out to a processor; most preferably athe selected third instant in time, the content of the respectivememory/storage means 107 of every pixel 101 in the pixel array isread-out to a processor.

For ease of understanding a time span before and including the selectedfirst instant in time shall be called the reset phase, the time betweenthe selected first instant in time and the selected third instant intime shall be called integration phase, and the time between theselected third instant in time until the content of all memory/storagemeans 107 has been read shall be called readout phase. The ensemble ofreset phase, integration phase and readout phase shall be called aframe.

FIG. 2a illustrates the vision sensor 1 of FIG. 1 in use. SpecificallyFIG. 2a shows a depth sensor assembly 200 which comprises a projector201 and a camera 202, which comprises the vision sensor 1 of FIG. 1. Thevision sensor 1 further comprises a processor 204. The depth sensorassembly 200 further comprises a memory 203. The processor 204 isconnected to the projector 201 and to the pixel array 100 of the visionsensor 1. Specifically, the processor 204 is connected to the memory 107of each pixel 101 in the pixel array 100 so that counter values storedin the memory 107 of each pixel can be selectively read out to theprocessor 204.

The projector 201 is configured to project electromagnetic rays (hereinafter referred as light rays) in a spectrum, on a surface 208; the lightrays may be reflected by the surface 208 and may be captured by thecamera 202. I should be noted that the light rays may be invisible tothe human eye e.g. infrared or ultraviolet light rays.

The projector 201 may be modelled using a pinhole model in which alllight rays projected by the projector 201 pass through a single point ormay be extrapolated to pass through a single point; said single pointdefines the optical centre 201 a of the projector 201. If the projectoruses a lens (e.g. a beamer), the according pinhole model may be derivedusing structured-light projector calibration methods known in the art.If the projector contains one or multiple other lens-free light sources(e.g. a laser), the projector may be configured such that all projectedlight rays pass through a single point or may be extrapolated to passthrough a single point which corresponds to the optical centre 201 a ofthe projector 201. The virtual image plane 201 b of the pinhole model ofthe projector 201 is oriented perpendicular to the principal axis 201 cof the projector 201 and is located at a distance from optical centre201 a of the projector 202 which is modelled as the focal length (f) ofthe projector 201.

Equivalently, the camera 202 may be modelled using a pinhole model inwhich all light rays indecent on the camera 202 pass through a singlepoint or may be extrapolated to pass through a single point; said singlepoint defines the optical centre 202 a of the camera 202. The virtualimage plane 202 b of the pinhole camera model of the camera 202 isoriented perpendicular to the principal axis 202 c of the camera 202 andis located at a distance from optical centre 202 a of the camera 202which is equal to the focal length (f) of the camera 202.

The light rays which are projected by the projector 201, and which aresubsequently incident on the vision sensor 1 of the camera 202 (e.g.light rays which are projected by the projector 201 and are reflected bya surface 208 towards the vision sensor 1 of the camera 202, so thatthey are incident on the vision sensor 1 of the camera 202) may bemodelled as an image on the virtual image plane 201 b of the pinholemodel of the projector; said image on virtual image plane 201 b isreferred to hereinafter as the projection image.

Calibration techniques known in the art can be used to find the pinholemodel of camera 202 including the optical centre 202 a of the camera202, the principal axis 202 c of the camera 202, focal length (f) of thecamera 202 and the virtual image plane 202 b of the camera 202.

The projector 201, and camera 202 are positioned at a fixed position andorientation relative to one another. The baseline 205 is a referenceline which intersects both the optical centre 201 a of the projector 201and the optical centre 202 a of the camera 202.

Since the projector 201, and camera 202 are at a fixed position relativeto one another, the distance between the optical centre 202 a of theprojector 201 and the optical centre 202 a of the camera 202 is alsofixed. The baseline distance between the optical centre 202 a of theprojector 201 and the optical centre 202 a of the camera 202 is denotedin FIG. 2a as ‘b’. The baseline distance ‘b’ between the optical centre202 a of the projector 201 and the optical centre 202 a of the camera202 can be determined using techniques known in the art.

The light which a respective pixel receives when the projector 201 isnot projecting light onto the surface 208 is referred hereafter as the“background illumination”. At a first instant in time, before theprojector 201 is operated to project any light rays onto the surface208, each pixel in the vision sensor 1 is operated to sample theirrespective background illumination (i.e. each pixel 101 is operated tosample the intensity of the light which is incident on the pixel beforethe projector 201 is operated to project light). In each pixel, in orderto sample their background illumination, the current source 103 in thepixel is configured to output a current which has a constant currentlevel which is equal to the current level of the current output by thephotosensor 102. Preferably, also at the first instant in time beforethe projector 201 is operated to project any light onto the surface 208the counter 108 is set to a starting value (e.g. ‘zero’). The currentsource 103 is set to maintain said constant current level.

Once each pixel 101 has sampled their respective backgroundilluminations and the counter 108 has been set to a starting value, theprojector 201 is then operated to project light rays onto a surface 208so as to form a projected light pattern on the surface 208. At leastsome of the projected light rays will be reflected by the surface 208.The projector 201 and camera 202 are orientated such that at least someof the projected light rays which are reflected by the surface 208 willbe incident on one or more of the pixels 101 in the pixel array 100 ofthe vision sensor 1 in the camera 202.

After the first instant in time, when the projector 201 begins toproject light rays onto a surface 208 to form a projected light pattern,the counter 108 is initiated to begin counting from its starting value.Also at the instant the projector 201 begins to project light rays ontoa surface 208 to form a projected light pattern, the integrators 104 ofthe respective pixels 101 in the pixel array 100 will begin to integratethe difference between the level of current output by the current source103 and the level of current output by the photosensor 102 of thatpixel. It should be understood that the current source 103 in the pixelmaintains a constant current output from when the backgroundillumination was sampled, but the current output by the photosensor 102of that pixel will vary depending on the light incident on the pixel;typically the current output by the photosensor 102 will increase whenthe projector 201 projects light onto the surface 208 as some of thelight will be reflected from the surface 208 and will be incident on thepixels in the pixel array.

It will be understood that the projected light pattern may take anysuitable form; for example, the projected light pattern may be a line(such as a vertical line), or, a random dot pattern.

In the most preferred embodiment the pixels 101 in the pixel array 100lie on a single plane; the projector 201 and camera 202 are positionedrelative to one another, such that: the baseline 205 (i.e. the linewhich intersects both the optical centre 201 a of the projector 201 andthe optical centre 202 a of the camera 202) is perpendicular to aprincipal axis 202 c of the camera 202 and is parallel to the singleplane on which all pixels 101 in the pixel array 100 lie, and is alignedwith one of the two coordinates of the pixel array 100, and such thatthe baseline 205 and the principal axis 202 c of the camera lie on thesame plane (hereinafter referred as the baseplane 210). It should benoted that in FIG. 2a , the baseplane 210 is parallel to the plane ofthe page.

In this example, the projector 201 projects a plurality of light rayswhich define a line of light on the surface 208, (i.e. the projector 201projects a plurality of light rays which define a line pattern of lighton the surface 208); in this example shown in FIGS. 2a-b , the baseline205 has a horizontal orientation in space, and the projector 201projects a vertical line 209 onto the surface 208 (i.e. the projectedline 209 is perpendicular to the baseline 205).

To project a line of light on the surface 208 the projector 201 projectsa plurality of light rays all of which lie on a single plane(hereinafter referred as the line projection plane). The projected line209 shall be oriented such than the line projection plane isperpendicular to the baseplane 210; the angle between the baseline 205and the line projection plane is referred to hereafter as projectionangle γ. The projection angle γ may be increased or decreased over timein order to scan the projected line pattern across the surface 208.

If the projected pattern is a vertical line that is perpendicular to thebaseplane 210 as in the most preferred embodiment, then (assuming apinhole model for the camera 202) the corresponding epipolar lines onthe virtual image plane 202 b of the camera 202 will run horizontallyand parallel to one another. This allows to unambiguously assign acaptured point in the camera to an epipolar plane which would not bepossible if the epipolar lines would cross in the field of view of thecamera.

As mentioned in the example illustrated in FIG. 2a , the projector 201is configured to project a vertical line 209 onto the surface 208; theprojection angle (γ) of the projector 201 is increased (over time) toscan that vertical line 209 across the surface 208 (i.e. γ=f(t)) sothat, ideally, every point on the surface 208 will have received a lightray which was projected by the projector at some point in time. Itshould be noted that the projection angle (γ) is the angle which theline projection plane (i.e. the plane on which all of the plurality oflight rays which form the vertical line lie) forms with the base line205. Importantly, since the plurality of projected light rays define avertical line 209, each of the plurality of projected light rays willall lie on the same line projection plane, thus each of the plurality ofprojected light rays will each form the same angle with the base line205, and thus each of the plurality of projected light rays will havethe same projection angle (γ).

It should be noted that in this embodiment the projected pattern isfixed; in other words at each projection angle (γ) at which theprojector 201 projects, the same vertical line 209 pattern will beprojected (although the appearance of the projected pattern on thesurface 208 will depend on whether the surface 208 is flat orcontoured).

Specifically, in this embodiment shown in FIG. 2a , at a first knowntime instant the light rays which define the vertical line 209 on thesurface 208 are projected at a known, initial, predefined projectionangle (γ). Then the projection angle (γ) is increased at a known,predefined rate; specifically, in this example the projection angle (γ)is increased a predefined amount each time the counter value increasesby a predefined amount (i.e. each time the value of the counter 108 inthe vision sensor 1 increases by a predefined amount); thus, projectionangle (γ) is proportional to the counter value on the counter 108. Forexample the known, initial, predefined projection angle may be ‘0’degrees and the projection angle may be increased ‘1’ degree per counterincrement; thus the projection angle (γ) of the plurality of projectedlight rays which define the vertical line 209 on the surface 208, isequal to the counter value; or initial, predefined projection angle maybe ‘0’ degrees and the projection angle (γ) may be increased ‘2’ degreesper counter 108 increment; thus the projection angle (γ) of theplurality of projected light rays which define the vertical line 209 onthe surface 208, is double the counter value.

A formula defining the projection angle (γ) as a function of countervalue is stored in the memory 203.

When the projector 201 projects the vertical line 209 on the surface 208at least some of the projected light rays will be reflected by thesurface 208. At least some of the projected light rays which arereflected by the surface 208 will be incident on one or more of thepixels 101 in the pixel array 100 of the vision sensor 1 in the camera202.

In the example illustrated in FIG. 2a , at the first known time instantthe light rays, which define the fixed pattern (vertical line), areprojected at the initial, known, predefined projection angle (γ), ontothe surface 208; some of those projected light rays will be reflected bythe surface 208 and will be incident on one or more pixels 101 in thepixel array 100. The projection angle (γ) will be then increased, andthe projector 201 will project light rays, which define the fixedpattern (vertical line), onto a different position of the surface 208;some of those projected light rays will be reflected by the surface 208and will be incident on one or more other pixels 101 in the pixel array100 etc.

When a pixel 101 in the pixel array 100 receives a reflected light raythis will drive the integration value i.e. the integration of thedifference between the level of current output by the current source 103(which is the constant level to which is was set during the backgroundsampling) and the level of current output by the photosensor 102 of thatpixel 101 over the predefined threshold level and the counter value(i.e. the value on the counter 108) will be saved in the memory 107 ofthat pixel 101. Thus the counter value stored in the memory 107 of thepixel 101 corresponds to when a reflected light ray was incident on thatpixel 101.

In the pinhole camera model, a reflected light ray which is incident ona pixel 101 in the pixel array 100 must have a specific spatialdirection (in other words each pixel 101 in the pixel array 100 can onlyreceive reflected light rays which have a single specific spatialdirection (said single specific spatial direction will be different foreach pixel 101 in the pixel array 100); reflected light rays which donot have said single specific spatial direction will not be incident onthe pixel). The respective specific spatial direction for eachrespective pixel 101 in the pixel array is a three-dimensionaldirection; the three-dimensional direction can be projected onto thebaseplane 210, and the angle which said projected direction forms withthe baseline 205 defines an inclination angle (δ) for that pixel.

Thus, the inclination angle (δ) for a respective pixel 101 in the pixelarray 100 is the angle which, the projection of a reflected light rayonto the baseplane 210 must form with the baseline 205, in order forsaid reflected light ray to be incident on said pixel 101. Theinclination angle (δ) of the pixel 101 is dependent on the position ofthe pixel 101 within the pixel array 100, and position and orientationof the camera 202. In this embodiment the orientation and position ofthe camera is fixed, therefore the respective inclination angles (δ) ofeach pixel 101 in the pixel array 100 are fixed.

In one embodiment the inclination angle (δ) of each pixel 101 in thepixel array 100 is determined in a calibration step and the respectiveinclination angle (δ) of each pixel 101 is stored in the memory 203.

In the most preferred embodiment (as well as in the case of a calibratedcamera with a principal axis 202 c perpendicular to the baseline 205,the inclination angle (δ) of a pixel 101 in the pixel array 100 isdetermined using the following formula:

$\delta = {\frac{\pi}{2} - {\tan^{- 1}\left( \frac{d_{x}}{f} \right)}}$

Wherein f is the focal length of the camera 202, and d_(x) is the axisintercept of the axis along the baseplane 210 in a pixel directionvector d_(uv) connecting the optical centre of the calibrated camera tothe according pixel on the virtual image plane 202 b. Since in the mostpreferred embodiment, the x-axis coordinate of the direction vector isparallel to the baseplane 210 and since the principal axis 202 c isperpendicular to the baseline, the projection of the direction vectoronto the baseplane 210 is equivalent to the x component d_(x) and the zcomponent f of the direction vector.

The pixel direction vector d_(uv), for a given a point on the virtualimage plane 202 b with coordinates u and v, is represented as follows:

$d_{uv} = \begin{bmatrix}d_{x} \\d_{y} \\f\end{bmatrix}$

The pixel direction vector d_(uv) is determined using the followingformula:

d _(uv) =K ⁻¹Undistort(p)

Wherein p is the position of the respective pixel 101 in the pixel array101, and K is the “camera intrinsic matrix” so K⁻¹ is the inverse of the“camera intrinsic matrix”:

${p = \begin{bmatrix}u \\v \\1\end{bmatrix}},{K = \begin{bmatrix}f_{u}^{\prime} & s & c_{u} \\0 & f_{v}^{\prime} & c_{v} \\0 & 0 & 1\end{bmatrix}}$

Wherein p is represented in homogeneous coordinates.

Wherein with respect to the entries in the “camera intrinsic matrix” K,the metric focal length f is measured in meters, the intrinsiccalibration focal length f′ is measured in pixel dimensions such thatf′=f/(Ip), whereas Ip is the size of a pixel in the vision sensor 1measured in meters along the u- and v-coordinates (noted by theaccording subscripts); and wherein s is a skew parameter and c_(u) andc_(v) are coordinates of the principal point. The entries in the “cameraintrinsic matrix” K are preferably obtained in a calibration step inwhich the intrinsic parameters of the camera are determined using knowntechniques.

And wherein Undistort( ) is an image undistortion function whichcompensates distortion effects such as the radial lens distortion or thetangential distortion in the camera 201. The undistortion function maybe numerically approximated using the distortion parameters obtained ina calibration step in which the intrinsic parameters of the camera aredetermined using known techniques. The undistortion function ispreferably determined in a calibration step using known techniques.

After the vertical line 209 has been scanned across the surface 208(i.e. after the projector 201 has projected the vertical line 209 at afinal, maximum, projection angle), the respective counter values whichare stored in the respective memories 107 of each of respective pixel101 in the pixel array 100 are read out to the processor 204.

The processor 204 generates a counter image using said counter values.FIG. 2b illustrates an exemplary counter image 230 which the processor204 may generate; the counter image 230 is a matrix containing all ofsaid counter values which were stored in the respective memories 107 ofthe respective pixels 101, and wherein the position of each countervalue in the matrix corresponds to the position of the respective pixel101 in the pixel array 100. In FIG. 2b the counter values range from thelowest counter value t1 to the highest counter value te.

For each of the respective counter values which is provided in thecounter image 230, the processor 204 determines a respective depth valued using the following formula:

$d = {b\frac{{\tan (\gamma)} \cdot {\tan (\delta)}}{{\tan (\gamma)} + {\tan (\delta)}}}$

wherein the depth value d is the distance between, the point on thesurface 208 at which the reflected light ray which was incident on thepixel 101 (i.e. the pixel from whose memory 107 said counter value inquestion was read out) was reflected, and the baseline 205 measuredalong the principal axis 202 c of the camera 202; δ is the inclinationangle (δ) for said pixel 101 (i.e. the pixel from whose memory 107 saidcounter value in question was read out); and γ is the projection angle(γ) of the plurality of light rays projected by the projector 201 whichdefine the vertical line 209, at time the counter 108 had a countervalue corresponding to the counter value which was read out from thememory 107 of said pixel (the projection angle (γ) may be computed usingthe formula defining the projection angle (γ) as a function of countervalue which is stored in the memory 203); and ‘b’ is the baselinedistance between the optical centre 202 a of the projector 201 and theoptical centre 202 a of the camera 202.

It should be understood that in the embodiment in which the inclinationangle (δ) of each pixel 101 is determined in a calibration step and therespective inclination angle (δ) of each pixel 101 is stored in thememory 203, the processor 204 may simply retrieve the inclination angle(δ) for the respective pixel 101 (i.e. the pixel from whose memory 107said counter value in question was read out) from the memory 203. Inanother embodiment, the processor 204 may determine the inclinationangle (δ) for the respective pixel 101 (i.e. the pixel from whose memory107 said counter value in question was read out) using the formulaedescribed earlier in the description.

The baseline distance ‘b’ between the optical centre 202 a of theprojector 201 and the optical centre 202 a of the camera 202, can becalculated using known techniques in the art, and is typicallycalculated in a calibration step and it subsequently stored in thememory 203. Thus the baseline distance ‘b’ can be retrieved by theprocessor 204 from the memory 203.

As mentioned, the projection angle (γ) of the plurality of light raysprojected by the projector 201 and which define the vertical line 209 onthe surface 208, for any counter value (i.e. for any of the respectivecounter values which were read out from the memories 107 of respectivepixel 101 in the pixel array 100) can be determined using the formuladefining the projection angle (γ) as a function of counter value whichis stored in the memory 203. The processor 204 retrieves from the memory203 said formula defining the projection angle as a function of countervalue; for each pixel, the processor enters the counter value which wasread out from the memory 107 of said pixel into the formula to determinethe projection angle (γ) of the plurality of light rays projected by theprojector 201 which defined the vertical line 209 on the surface 208, atthe time when the counter 108 had a counter value corresponding to thecounter value which was read out from the memory 107 of said pixel.

Thus for each of the respective counter values which is provided in thecounter image 230, the processor 204 can determine a respective depthvalue d using the following formula:

$d = {b\frac{{\tan (\gamma)} \cdot {\tan (\delta)}}{{\tan (\gamma)} + {\tan (\delta)}}}$

The processor 204 then generates a depth map 231 using all of saiddetermined depth values d. FIG. 2b illustrates a depth map 231; thedepth map 231 is a matrix containing all of said determined depth valuesd, and wherein the position of each depth value d in the matrixcorresponds to the position of the corresponding counter value in thecounter image 230 from which that respective depth value d wasdetermined. In the depth map 231 of FIG. 2b the depth values shown arein a range between the minimal distance d− and the maximal distance d+.

In the above-mentioned embodiment, the projector 201 projects a verticalline 209 onto the surface 208 and the projection angle (γ) is increasedat a predefined rate to scan that vertical line 209 across the surface208. In another embodiment, as will be now described, instead ofprojecting a vertical line 209, a light pattern which comprises aplurality of distinct features (geometric primitives or combinations,such as dots, edges or lines) may be projected onto the surface 208;said light pattern is such that the corresponding epipolar lines of theprojected features which define the light pattern on the surface 208, donot overlap or cross within the field of view of the camera 202. For anyfeature captured by the camera, said projected pattern allowsestablishing an unambiguous correspondence with a projected featureusing epipolar geometry and therefore said projected pattern shall bereferred to hereinafter as an unambiguous pattern.

The vertical line projected in FIG. 2 is one of many possibleunambiguous patterns given the projector 201 and the camera 202 arearranged horizontally.

For the highest triangulation accuracy, the projected features wouldideally be infinitesimally narrow but limitations in the projector andthe camera result in features that cover a certain area when projectedonto a surface 208. Therefore, each feature shall be associated with ananchor point that allows to associate features, that may span multiplepixels in the pixel array, with a distinct point. For a projected dot,said anchor point may be its centre.

To determine an unambiguous pattern, the process of stereo imagerectification known in the art is applied. Hereby the fact that inrectified images, the epipolar lines run parallel along one of the twoimage coordinates (the u* coordinate) is exploited:

Step 1. Calibrate the intrinsic camera calibration parameters of theprojector 201 and the camera 202 using projector and camera calibrationprocedures known in the art. Calibrate the extrinsic camera calibrationparameters using stereo calibration methods known in the art. Computethe projective transformations needed for the image rectification fromthe camera coordinates to the rectified coordinates using rectificationmethods known in the art.

Step 2. Compute the inverse to the projective transformations and theinverse of the undistortion function (wherein the undistortion functionis obtained in a camera calibration step using known methods). Note thatthe inverse to the radial component of the undistortion function (apolynomial function) must be numerically approximated.

Step 3. Chose a suitable light pattern such that only one distinctvisual feature, such as a dot, line or edge, in the rectified projectionpattern is present per v* coordinate, wherein v* refers to the rectifiedimage coordinate perpendicular to the epipolar lines.

Step 4. Apply the inverse of the projective transformation of theprojector, obtained in Step 2, to the rectified projection pattern. Thenapply the inverse to the undistortion function, obtained in Step 2, toobtain the unambiguous pattern that shall be projected.

Most preferably steps 1-4 will be carried out a plurality of times todetermine a plurality of different unambiguous patterns.

For the computation of the depth map, the data acquisition is similar tothe procedure described with respect to FIGS. 2a and b:

The light which a respective pixel receives when the projector 201 isnot projecting light onto the surface 208 is referred hereafter as the“background illumination”. Initially, before the projector 201 isoperated to project any light rays onto the surface 208, each pixel inthe vision sensor 1 is operated to sample their respective backgroundillumination (i.e. each pixel 101 is operated to sample the intensity ofthe light which is incident on the pixel before the projector 201 isoperated to project light). In each pixel, in order to sample theirbackground illumination the current source 103 in the pixel isconfigured to output a current which has a constant current level whichis equal to the current level of the current output by the photosensor102. Preferably, also initially, before the projector 201 is operated toproject any light onto the surface 208 the counter 108 is set to astarting value (e.g. ‘zero’). The current source 103 is set to maintainsaid constant current level.

Once each pixel 101 has sampled their respective backgroundilluminations and the counter 108 has been set to a starting value, thefollowing steps are performed:

Step A. Project, using the projector 201, a first unambiguous pattern(obtained by carrying out steps 1-4), onto the surface 208. The firstunambiguous pattern which was projected is stored in the memory 203 (ora parametrized form of the first unambiguous pattern is stored in thememory 203) together with the value of the counter 108 at the timeinstant when the projector 201 projected said first unambiguous lightpattern; wherein the first unambiguous pattern is stored in the memory203 in association with the value of the counter 108.

At the instant the projector 201 first begins to project the firstunambiguous pattern the counter 108 is initiated to begin counting fromits starting value. Also at the instant the projector 201 begins toproject first unambiguous pattern, the integrators 104 of the respectivepixels 101 in the pixel array 100 will begin to integrate the differencebetween the level of current output by the current source 103 and thelevel of current output by the photosensor 102 of that pixel. It shouldbe understood that the current source 103 in the pixel maintains aconstant current output from when the background illumination wassampled, but the current output by the photosensor 102 of that pixelwill vary depending on the light incident on the pixel; typically thecurrent output by the photosensor 102 will increase when the projector201 projects light onto the surface 208 as some of the light will bereflected from the surface 208 and will be incident on the pixels in thepixel array.

When the projector 201 projects the first unambiguous pattern (obtainedby carrying out steps 1-4), onto the surface 208, at least some of theprojected light rays will be reflected by the surface 208. The projector201 and camera 202 are orientated such that at least some of theprojected light rays which are reflected by the surface 208 will beincident on one or more of the pixels 101 in the pixel array 100 of thevision sensor 1 in the camera 202.

When a pixel 101 in the pixel array 100 receives a reflected light raythis will drive the integration value (i.e. the integration of thedifference between the level of current output by the current source 103(which is the constant level to which is was set during the backgroundsampling) and the level of current output by the photosensor 102 of thatpixel 101 over the predefined threshold level and the counter value(i.e. the value on the counter 108) will be saved in the memory 107 ofthat pixel 101. Thus, the counter value stored in the memory 107 of thepixel 101 corresponds to when a reflected light ray was incident on thatpixel 101.

Step B. After the value on the counter 108 has increase by a predefinedamount, then project a second unambiguous pattern (e.g. by moving orrotating the previous pattern or by creating a new pattern according tothe procedure to create an unambiguous pattern) onto the surface 208.Preferably the second unambiguous pattern will be incident on differentareas of the surface 208 to which the first unambiguous pattern wasincident so that different areas of the surface 208 are illuminatedcompared to when the first unambiguous pattern was projected. Store thesecond unambiguous light pattern in the memory 203 (or a parametrizedform of the second unambiguous pattern) together with the value of thecounter 108 at the time instant when the projector 201 projected saidsecond unambiguous pattern; wherein the second unambiguous pattern isstored in the memory 203 in association with said value of the counter108.

Step C. Preferably, repeat step B a plurality of times, each timeprojecting a different respective unambiguous pattern (e.g. repeat stepB four more times, respectively projecting a third, fourth, fifth, sixthunambiguous pattern etc.) until a predefined spatial and temporalresolution of the data acquired by the vision sensor 1 is achieved. Itshould be noted that step B and C are optional, it is sufficient for theinvention that only the first unambiguous pattern is projected.

Step D. After the respective unambiguous pattern has been projected bythe projector 201, the respective counter values which are stored in therespective memories 107 of each of respective pixel 101 in the pixelarray 100 are read out to the processor 204. The processor 204 generatesa counter image using said counter values (similar to the counter image230 shown in FIG. 2b ); the counter image is a matrix containing all ofsaid counter values which were stored in the respective memories 107 ofthe respective pixels 101, and wherein the position of each countervalue in the matrix corresponds to the position of the respective pixel101 in the pixel array 100. The counter image is then rectified usingthe projective transformation derived from the calibration steps for thestereo rectification known in the art to provide a rectified counterimage.

Step E. For each pixel in the rectified counter image perform followingsteps:

-   -   a. Determine whether a counter value has been stored, otherwise        skip the pixel. (there will be no counter value stored in a        pixel if that pixel has not received sufficient light to allow        the integration by the integrator 104 of that pixel to reach the        predefined threshold; this will be the case for example, if        after the plurality of different respective unambiguous patterns        have been projected, that no reflected light ray has been        incident on that pixel)    -   b. Determine if any neighbouring pixels (i.e. pixels which are        adjacent to the pixel) contain the same counter value;        identifying clusters of pixels, each cluster composed of a        plurality of adjacent pixels each of which comprise the same        counter value (each cluster may comprise one or more pixels).    -   c. Compute the anchor point p_(c)* (in rectified camera        coordinates) of the feature associated with said cluster of        pixels (that may just contain a single pixel). E.g. for a        projected dot, determine the centre of said cluster of pixels    -   d. Identify the counter value common to said cluster of pixels        (that may contain one or more pixels); retrieve from the memory        203 the unambiguous pattern which is associated in the memory        203 with said identified counter value.    -   e. If the unambiguous pattern is not stored in its rectified        form, rectify the pattern.    -   f. Determine the rectified coordinate v_(c)* (which is        perpendicular to the epipolar lines) of the anchor point p_(c)*        to search the according epipolar line of the unambiguous pattern        for the corresponding projected feature and its associated        anchor point p_(p)*.    -   g. Use the coordinates u* along the rectified epipolar lines of        the points associated with the projected and the captured        feature to compute the disparity. The disparity can then be used        to compute the depth d according to:

$\begin{matrix}{d = {\frac{bf}{disparity} = \frac{bf}{u_{p}^{*} - u_{c}^{*}}}}\end{matrix}$

-   -   wherein d is the depth (distance between point on surface and        baseline measured along the direction of the principal axis of        the camera), and ‘b’ is the baseline distance between the        optical centre 202 a of the projector 201 and the optical centre        202 a of the camera 202, f is the focal length of the camera        202, and u_(p)* and u_(c)* are the rectified coordinates along        the epipolar lines of a appoint associated with a distinct        feature in the rectified projection image and the rectified        camera image.

The processor then generates a depth map using all of said determineddepth values d. The depth map is a matrix containing all of saiddetermined depth values d, and wherein the position of each depth valued in the matrix corresponds to the position of the corresponding pixelin the rectified camera image.

In the above embodiment, the projector 205 is configured to project aseries of unambiguous patterns (such as a series of dot patterns), eachpattern in the series formed by light rays. The patterns within theseries of patterns are such that the corresponding epipolar lines withinthe camera do not cross or overlap within the captured camera images. Inanother embodiment, the series of unambiguous patterns are such thatafter the series has been projected, every point on the surface 208preferably will have received a light ray which was projected by theprojector at some point in time. Advantageously, this embodiment allowshigh spatial resolution i.e. allows to determine the distance from manypoints on the surface 208 to the baseline 205.

In another embodiment, the series of unambiguous patterns are such thatafter the series has been projected, only some points on the surface 208will have received a light ray which was projected by the projector.While this embodiment achieves lower spatial resolution itadvantageously allows to quickly compute the distance from some pointson the surface 208 to the baseline 205.

In another embodiment, the series of unambiguous patterns are such thatafter all of the patterns in the sequence have been projected, therespective counter values which are stored in the respective memories107 of each of respective pixel 101 in the pixel array 100 are read outto the processor 204.

In yet a further embodiment the projected pattern (or the patterns in aseries of projected patterns) are not constrained in any way withrespect to their corresponding epipolar geometry at the camera (e.g.said light pattern may be such that the corresponding epipolar lines ofthe projected features which define the light pattern on the surface 208do overlap or cross within the field of view of the camera 202):

For example, in one further embodiment the projector 201 projects arandom feature pattern (such as the random dot patterns used in KinectV1) onto the surface 208; the random feature pattern is modulated inintensity, moved or projected in a series of random feature patternssuch that the pixels 101 in the camera 202 are exposed to increasedilluminations that trigger the storage of the counter value. Due to thefact that even in the rectified projection pattern multiple features perepipolar line are present, ambiguities for matching counter values inthe counter image captured by the camera 202 to projected featuresarise. To resolve these ambiguities, spatial cues and local correlationson the relative distribution of the features may be used. So instead ofmatching individual features along the epipolar lines, sets of featuresare matched. The methods used for establishing said correlations mayfollow the methods outlined in U.S. Pat. No. 8,493,496 B2.

In another example, the projector 201 projects a pattern that comprisesof a sequence of one-dimensional masks arranged in a two-dimensionalpattern that may be used to compute the depth with increasingly complexmasks (such as the binary search tree patterns in the Intel RealSense).Starting off with an unambiguous mask, the matching of the projected andcaptured image of the mask may for subsequent masks be constrained andtherefore simplified. The according projector may follow US 20120218464A1.

It should be noted that the steps described in any of the embodimentsdescribed in this application, could be repeated one or a plurality oftimes so as to achieve a plurality of depth maps.

It should be noted that in any of the embodiments described in thisapplication the counter value of the counter 108 may be digital value.In one embodiment, the digital value of the counter 108 is stored in thememory 107 of the pixel 101 when the integration value (i.e. theintegration of the difference between the level of current output by thecurrent source 103 (which is the constant level to which is was setduring the background sampling) and the level of current output by thephotosensor 102 of that pixel 101) is equal to or greater than thepredefined threshold level. In another embodiment, the counter value isconverted to an analogue value which is representative of the digitalcounter value of the counter; and that analogue value is stored in thememory 107 of the pixel. The analogue value may be converted back to adigital value before is it read out the processor 204. Advantageouslystoring an analogue value which is representative of the digital countervalue requires less pixel area than storing a digital counter value.

Advantageously, as can be derived from the exemplary embodimentsdescribed above, the use of a vision sensor 1 according to the presentinvention provides a depth map which can be updated at a high framerate. The frame rate depends directly on the sensor's temporalresolution. In other words, because the vision sensor can detect achange in light intensity (i.e. a change in the intensity of lightincident on the pixels in the pixel array) relative to the backgroundillumination, at a high time resolution, the frame rate of the depth mapcan be high.

Advantageously, in the present invention, most of the computations forthe depth computation are very simple and fast. No matching or searchingsteps are required. The inclination angles for each of the pixels 101 inthe pixel array 100 may be pre-computed and stored in the memory 203 (inthe form of look-up tables for example). Also in the present invention,the computation of the projection angles is simplified (e.g. using aknown initial projection angle, and a known rate of increase ofprojection angle).

By projecting patterns for which the epipolar lines do not cross oroverlap within the captured camera images (i.e. unambiguous patternssuch as a vertical line pattern), the underlying matching of projectedand received points is simple and unambiguous. It works even withcomplex surfaces and does not require any assumptions on the continuityof the surface.

Advantageously, by providing storage means integral to each respectivepixel enables accurate recording of the value on the counter at the timeinstant when the integration value (as determined by the integrator) ofthat respective pixel, reaches the predefined threshold. In particular,it enables accurate recording of the value on the counter at the timeinstant when the integration value (as determined by the integrator) ofthat respective pixel, reaches the predefined threshold, even if aplurality of the pixels in the pixel array simultaneously reach thepredefined threshold. The ability to accurately record the value on thecounter at the time instant when the integration value (as determined bythe integrator) of that respective pixel, reaches the predefinedthreshold, allows to more rapidly alternate projected predefined patternover the surface S, and thereby acquire the depth map faster. Theadvantages of acquiring the depth map faster are that it reduces motionartefacts and allows having a high update rate on the depth map.

Additionally, integrating the difference between the backgroundillumination and the instantaneous illumination of the pixel, in eachrespective pixel in the pixel array (i.e. using respective integratorswhich are integral to respective pixels), allows operation to beindependent of ambient illumination; thus, there is reduced risk of thepixels in the pixel array becoming saturated.

Furthermore, in one embodiment, the projector can be configured toproject a plurality of different predefined patterns; a user will selectone of the plurality of different predefined patterns for the projectorto project. Also, a plurality of different predefined thresholds isprovided; the user will select one of the plurality of predefinedthresholds. Most preferably the user will select one of the plurality ofpredefined thresholds depending on the predefined pattern which theprojector is to project (e.g. a first predefined threshold is used ifthe projector is to project a first predefined pattern, and a secondpredefined threshold is used if the projector is to project a secondpredefined pattern); the possibility to select different predefinedthresholds for different projection patterns, allows a goodsignal-to-noise ratio without the need to use a multi-bit analogue todigital converter.

It should be understood that a switch may be in two states. In a firststate the two terminals of the switch are electronically connected whichis hereinafter referred as closed and in a second state the twoterminals are not electronically connected which is hereinafter referredas open. An electronical connection may be implemented as directelectronic connection that allows electrons to flow from one terminal tothe other. Alternatively an electronical connection may be implementedusing voltage buffers.

It should be understood that the photosensor 102 provided in each pixel101 of the pixel array 100 of the vision sensor 1 of FIG. 1a , may takeany suitable configuration. As shown in FIG. 1b , the photosensor 102comprises an output 102 a. FIGS. 4a and 4b provide two exemplaryimplementations of the photosensor in each pixel.

Referring to FIG. 4a , the photosensor 102 comprises a photodiode 400;an output 411 of the photodiode defines the output 102 a of thephotosensor.

Referring to FIG. 4b the photosensor 102 comprises a photodiode 401, acascode NMOS transistor 402 and an inverting amplifier 403. Thephotodiode 401 has an output 401 a, and the inverting amplifier 403 hasan input 403 a and an output 403 b. The output 401 a of the photodiode401 is connected to both the source of cascode NMOS transistor 402 andalso to the input of the inverting amplifier 403. The gate of cascodeNMOS transistor 402 is connected to the output of the invertingamplifier 403. The drain of the cascode NMOS transistor 402 defines theoutput 102 a of the photosensor 102.

It should be understood that the current source 103 provided in eachpixel 101 of the pixel array 100 of the vision sensor 1 of FIG. 1a , maytake any suitable configuration. As shown in FIG. 1b , the currentsource 103 comprises output 103 a. FIGS. 5a and 5b provide two exemplaryimplementations of the current source 103 in each pixel 101.

Referring to FIG. 5a , the current source 103 comprises a PMOStransistor MP1 501, a switch 502 and a capacitor 503. The capacitor isconnected between the gate of the PMOS transistor 501 and the powersupply 504. The capacitor 503 may be explicit or may be the parasiticcapacitance of the gate of the PMOS transistor 503. The switch 502 islocated between the gate of the PMOS transistor MP1 501 and an input 103b that is used to configure the current source. The switch 502 may beselectively put in a closed state to electronically connect the gate ofthe PMOS transistor MP1 501 and the input 103 b; or the switch 502 maybe put in an open state to provide an open circuit between the gate ofthe PMOS transistor MP1 501 and the input 103 b, which allows to set thecurrent output to a constant level.

When the pixels 101 of the pixel array 100 used in the depth sensor 200of FIG. 2a for example have a current source as depicted in FIG. 5a ,then a signal (Reset) which is provided by the processor 204 is used tocontrol the state of the switch 502; specifically the processor 204provides a signal (Reset) which controls the state of the switch 502which puts the switch 502 in a closed state or open state. In thecurrent source 103 shown in FIG. 5a the drain of the PMOS transistor MP1defines the output 103 a of the current source 103.

During the reset phase, the switch 502 is configured to be in the closedstate, during the integration phase, the switch is configured to be inthe open state.

Referring to FIG. 5b , the current source 103 comprises a PMOStransistor 511, a switch RS 512, a cascode PMOS transistor 513 and acapacitor 515. The capacitor is connected between the gate of the PMOStransistor 511 and the power supply. The capacitor 515 may be explicitor may be the parasitic capacitance of the gate of the PMOS transistor515. The switch RS 512 is located between the gate of the PMOStransistor MP1 511 and an input 103 b of the current source that is usedto configure the current source 103. The switch RS may be put in aclosed state to electronically connect the gate of the PMOS transistorMP1 511 and the input 103 b; or the switch RS may be put in an openstate to provide an open circuit between the gate of the PMOS transistorMP1 511 and the input 103 b.

When the pixels 101 of the pixel array 100 used in the depth sensor 200of FIG. 2a for example contain a current source according to FIG. 5b ,then a signal (Reset) which is provided by the processor 204 is used tocontrol the state of the switch 502 such that the backgroundillumination is sampled before projecting the first light pattern.Specifically the processor 204 provides a signal (Reset) which controlsthe state of the switch 502. During the reset phase, the switch isconfigured to be in the closed state to sample the backgroundillumination, during the integration phase, the switch is configured tobe in the open state.

The gate of the cascode PMOS transistor 513 is connected to a voltagesource 514. The drain of PMOS transistor 511 is connected to the sourceof the cascode PMOS transistor 513. The drain of the cascode PMOStransistor 513 defines the output 103 a of the current source 103.

As shown in FIG. 1b the output 103 a of the current source 103 isconnected to the output 102 a of the photosensor 102 and to the input104 a of the integrator 104.

It should be understood that the integrator 104 provided in each pixel101 of the pixel array 100 of the vision sensor 1 of FIG. 1a , may takeany suitable configuration. FIGS. 6 to 9 provide example implementationsfor the integrator 104 in each pixel 101.

Referring to FIG. 6, the integrator 104 comprises a capacitor 600 havinga first terminal and second terminal; and an amplifier 601. The firstterminal of the capacitor 600 is connected to ground and the secondterminal 600 a is connected to the first input 601 a of the amplifier601 and also defines the input 104 a of the integrator so that thecapacitor 600 can integrate the difference in the current output fromthe current source 103 and current output from the photosensor 102. Thecapacitor 600 may be explicit or may be a parasitic capacitance of thecircuit elements connected to the input of the integrator 104 a. Thesecond input 601 b of the amplifier 601 is connected to a voltage source602. During the reset phase, the voltage source 602 supplies a firstvoltage level Vreset to the second input of the amplifier 601. Duringthe integration phase, the voltage source 602 is set to a second voltagelevel, so that the voltage source 602 supplies a second voltage level tothe second input of the amplifier 602; the second voltage level may belower than the first voltage level. The difference between the firstvoltage level Vreset and the second voltage level defines saidpredefined threshold level. The output 601 c of the amplifier 600defines the output 104 b of the integrator.

Referring to FIG. 7 which shows yet a further possible implementationfor the integrator 104; in this example the integrator 104 comprises atwo-stage common source amplifier. The integrator 104 comprises a firststage having a first PMOS transistor 701 and a first NMOS transistor703; and a second stage having a second PMOS transistor 702 and a secondNMOS transistor 704; the gate of first NMOS transistor 703 of the firststage is connected to a voltage source 602, the gate of the second NMOStransistor 704 is connected to a bias voltage 705. The gate of the firstPMOS transistor 701 defines the input 104 a of the integrator 104. Theparasitic capacitance of the first PMOS transistor 701 is used tointegrate the difference in the current output from the current source103 and current output from the photosensor 102 (i.e. is equivalent tothe capacitor 600 of the implementation shown in FIG. 6). The drain ofthe first PMOS transistor 701 is connected to the drain of the firstNMOS transistor 703 and to the gate of the second PMOS transistor 702.The drain of the second PMOS transistor 702 is connected to the drain ofthe second NMOS transistor 704. The node between the drain of the secondPMOS transistor 702 and the drain of the second NMOS transistor 704defines the output 104 b of the integrator 104.

Referring to FIG. 8 which shows yet a further possible implementationfor the integrator 104; in this example the integrator 104 comprises afive-transistor operational transconductance amplifier (OTA) 800. Thegate of the first input transistor 801 defines the input 104 a of theintegrator. The gate of the second input transistor 802 is connected tothe voltage source 602. The gate of the bias transistor 803 is connectedto a bias voltage source 804. The output 800 a of the OTA 800 definesthe output 104 b of the integrator 104.

Referring to FIG. 9 which shows yet a further possible implementationfor the integrator 104; in this example the integrator 104 comprises acapacitive amplifier 900 and a comparator 901. The comparator 901comprises a first and second input 901 a, 901 b and comprises an output901 c. The capacitive amplifier 900 comprises of a first capacitor 902comprising a first and second terminal 902 a, 902 b, a second capacitor903 comprising a first and second terminal 903 a, 903 b, an invertingamplifier 904 having an input 904 a and an output 904 b, a first switch915 a and a second switch 915 b. The first terminal 902 a of the firstcapacitor 902 defines the input 104 a of the integrator, and the secondterminal 902 b of the first capacitor 902 is connected to the input 904a of the inverting amplifier 904. The first terminal 903 a of the secondcapacitor 903 is connected to the second terminal 902 b of the firstcapacitor 902, and the second terminal 903 b of the second capacitor 903is connected to the output 904 b of the inverting amplifier 904. Theoutput 904 b of the inverting amplifier 904 is connected to the firstinput 901 a of the comparator 901. The second input 901 b of thecomparator 900 is connected to a first voltage source 602. The output901 c of the comparator 901 defines the output 104 b of the integrator104.

The first switch 915 a is connected between the first terminal 902 a ofthe first capacitor 902 and the output 904 b of the inverting amplifier904. The second switch 915 b is connected between the input 904 b of theinverting amplifier 904 and a second voltage source 907. The first andsecond switch 915 a,b are configured to be closed during the resetphase, and are configured to be open during the integration phase.

It should be understood that each pixel 101 of the pixel array 101 ofthe vision sensor 1 of FIG. 1a , may take any suitable configuration.FIG. 3 illustrates the preferred configuration of a pixel 101; it shouldbe understood that each pixel 101 in the pixel array 100 of the visionsensor 1 may be configured as illustrated in FIG. 3.

The pixel 101 shown in FIG. 3 comprises a photosensor 102 which isimplemented as a photodiode 400, and a current source 103 which isimplemented as a PMOS transistor 501 with the source connected to apower supply 504 and the drain connected to the output 102 a of thephotosensor 102.

The integrator 104 comprises an amplifier 601 and a capacitor 600 thatintegrates the difference between the current output by the currentsource 103 (which is set to a constant at the first time instant) andthe current output by the photosensor 102. The first terminal of thecapacitor is connected to ground. The amplifier 601 has a first input601 a, a second input 601 b and an output 601 c. The first input 601 aof the amplifier 601 is connected to the second terminal 600 a of thecapacitor and defines the input 104 a of the integrator; and the secondinput 601 b of the amplifier 601 is connected to a voltage source 602.

The capacitor 600 may be an explicit capacitor or the parasiticcapacitances of the circuit elements connected to this node (which arethe current source 103, the photosensor 102 and the first input 601 a ofthe amplifier 601). The input 103 b of the current source is connectedto the output 104 b of the integrator. When the pixels 101 of the pixelarray 100 are used in the depth sensor 200 of FIG. 2a , then the stateof the switch 502 may be controlled by a global signal (called Reset)which is provided by the processor 204.

Advantageously, if the input 103 b of the current source 103 isconnected to the output of the integrator 104 b, configuring the currentsource so that it outputs a current with a constant current level whichis equal to the current level of the current output by the photosensorat the first time instant is done through a feedback loop when theswitch 502 is closed. This feedback loop (consisting of the amplifier601, the current source and the photosensor) ensures that while theswitch 502 is closed, the voltage at the first input 601 a of theamplifier will approach the voltage at the second input 601 b of theamplifier, which, during the reset phase, is set to a first voltagelevel Vreset by means of the voltage source 602.

The input 103 b of the current source 103 may optionally be connected tothe output 103 a of the current source instead of the output of theintegrator 104.

Advantageously, if the input 103 b of the current source 103 isconnected to the output 104 b of the integrator, the feedback loop usedfor configuring the current source takes into account the offset voltageof the amplifier 601 and the threshold voltage mismatch of the PMOStransistor 501.

Advantageously, if the pixels in the pixel array of the vision sensor 1each have the configuration shown in FIG. 3, the vision sensor 1 candetect an increase and/or decrease in the light intensity of a pixel.

It should be understood that the storage means 107 provided in eachpixel 101 of the pixel array 100 of the vision sensor 1 of FIG. 1a , maytake any suitable configuration. In the embodiment of FIG. 1a thestorage means in each pixel 101 is a memory 107. The memory 107 may beimplemented by means of a set of Flip-Flops or latches where the numberof Flip-Flops per pixel is equal to the number of bits of the counter108; or the memory may be implemented as an analogue memory such as acapacitor.

FIG. 10 illustrates a vision sensor 1010 according to a furtherembodiment of the present invention. The vision sensor 1010 has many ofthe same features as the vision sensor 1 in FIGS. 1a,b and like featuresare awarded the same reference numbers.

The vision sensor 1010 further comprises a voltage digital-to-analogueconverter (DAC) 1000, which is connected to the counter 108; the voltagedigital-to-analogue converter (DAC) 1000 creates a voltage proportionalto the value of the counter 108. This voltage which is created by thevoltage digital-to-analogue converter (DAC) 1000 is distributed to allpixels 101 in the pixel array 100. The memory/storage means 107 of eachpixel 101 in the pixel array 100 comprises a capacitor 1001 having afirst electrode 1001 a and second electrode 1001 b, a switch 1003 havingfirst terminal 1003 a and second terminal 1003 b, and a buffer 1004. Thefirst electrode 1001 a of the capacitor 1001 is connected to ground, thesecond electrode 1001 b of the capacitor 1001 is connected to the firstterminal 1003 a of the switch 1003. The second terminal 1003 b of theswitch 1003 is connected to the voltage digital-to-analogue converter(DAC) 1000.

The state of the switch is controlled by the output 104 b of theintegrator 104: if the integral of the difference between the currentoutput by the photosensor 102 and the constant current output by thecurrent source 103 is below the predefined threshold, the switch 1003 isclosed. As soon as the integral of difference between constant currentoutput by the photosensor 102 and current output by the current source103 reaches the predefined threshold, the integrator 104 outputs asignal at its output 104 b which opens the switch 1003.

The second electrode 1001 b of the capacitor 1001 is connected to aninput 1004 a of a buffer 1004. An output 1004 b of the buffer 1004 ofany pixel may be selectively connected to one of a set ofanalogue-to-digital converters (ADCs) 1005 by means of a switch 1006.For example there may be one ADC per column of pixels 101 in the array100, and each pixel in this column may be selectively connected to asingle analogue-to-digital converter (ADCs) 1005. Theanalogue-to-digital converters (ADCs) may be used to convert the voltagestored on capacitor 1001 of a pixel 100 to a digital value, which thencorresponds to the counter value at the instant where in thecorresponding pixel, the integral of difference between current outputby the photosensor 102 and constant current output by the current source103 reaches the predefined threshold.

FIG. 11a illustrates a vision sensor 1101 according to a furtherembodiment of the present invention. The vision sensor 1101 in FIG. 11has many of the same features as the vision sensor 1 in FIG. 1a and likefeatures are awarded the same reference numbers. FIG. 11b illustratesthe features of a pixel 101 in the pixel array 100 of the vision sensor1011 of FIG. 11a ; the pixel 101 has many of the same features as thepixel 101 shown in FIG. 1b and like features are awarded the samereference numbers. It will be understood that all of the pixels 101 inthe pixel array 100 of the vision sensor 1101 of FIG. 11a have theconfiguration shown in FIG. 11 b.

The vision sensor 1101 of FIG. 11a further comprises a readout circuit1100 which is operable to read out counter values which have been storedin the respective storage means/memories 107 of the pixels 101 in thepixel array 101 to a processor (e.g. to the processor 204, when thevision sensor 1011 is used in a depth sensor 200 shown in FIG. 2a ) aspart of the readout phase. The readout circuit 1100 allows reading out(consecutively or simultaneously) all the counter values stored in allof the respective storage means/memories 107 of all of the pixels 101 inthe pixel array.

When the vision sensor 1101 shown in FIG. 11a is used in the depthsensor 200 of FIG. 2a , the readout circuit 1100 may comprise acommunication bus connecting the readout circuit 1100 to the processor204. The communication bus may sequentially be connected to therespective storage means/memories 107 of the pixels 101 in the pixelarray 101 of all pixels in the vision sensor, such that the content ofthe counter values stored in the respective storage means/memories 107can be transmitted to the processor 204 over said communication bus.

FIG. 12a illustrates a vision sensor 1200 according to a furtherembodiment of the present invention. The vision sensor 1200 in FIG. 12has many of the same features as the vision sensor 1 in FIG. 1a and likefeatures are awarded the same reference numbers.

Referring to FIG. 12a , the vision sensor 1200 further comprises areadout circuit 1100 which is operable to read out counter values whichhave been stored in the respective storage means/memories 107 of thepixels 101 in the pixel array 101 to a processor (e.g. to the processor204, when the vision sensor 1200 is used in a depth sensor 200 shown inFIG. 2a ) as part of the readout phase. In this example the readoutcircuit 1100 is configured to read out, simultaneously, all the countervalues stored in all of the respective storage means/memories 107 of allof the pixels 101 positioned on the same row of the pixel array 100; thedifferent rows of the pixel array 100 are read out consecutively. Inother words, all the counter values stored in all of the respectivestorage means/memories 107 of all of the pixels 101 positioned on thefirst row 1102 a of the pixel array 100 are read out simultaneously;then after all of the respective storage means/memories 107 of all ofthe pixels 101 positioned on the first row 1102 a of the pixel array 100are read out simultaneously, then all of the respective storagemeans/memories 107 of all of the pixels 101 positioned on the second row1102 b of the pixel array 100 are read out simultaneously; etc. untilall of the storage means/memories 107 of all of the pixels 101 in allrows 1102 a-b have been read out.

The memory/storage means 107 of each pixel 101 on the top row 1102 a ofthe pixel array is directly connected to the readout circuit 1100. Thememory/storage means 107 of each pixel 101 on a second row 1102 b of thepixel array is connected to the memory/storage means 107 of a respectivepixel 101 on the first row 1102 a. The memory/storage means 107 of eachpixel 101 on a third row 1102 c of the pixel array is connected to thememory/storage means 107 of a respective pixel 101 on the second row1102 b. The memory/storage means 107 of each pixel 101 on a fourth row1102 d of the pixel array is connected to the memory/storage means 107of a respective pixel 101 on the third row 1102 c. Counter values storedin the memory/storage means 107 of pixels 101 on the second, third andfourth row 1102 b-d can be passed to the along the rows to thememory/storage means 107 of pixels 101 on the first row 1102 a and fromthere to the read out circuit 1100.

It should be understand that also the bottommost row could be connectedto the readout circuit.

FIG. 12b shows a pixel 101 of the vision sensor 1200; it will beunderstood that all pixels 101 in the pixel array 100 of the visionsensor 1200 will be configured as shown in FIG. 12b . As shown in FIG.12a , each pixel 101 is connected to a neighbouring pixel 101 a which islocated on a lower row of the pixel array, and to a neighbouring pixel101 b located on an upper row of the pixel array. It will be understoodthat those pixels located on the lower-most rows in the pixel array willonly be connected to respective neighbouring pixels 101 b located on aupper row of the pixel array and those pixels located on the upper-mostrows in the pixel array will only be connected to respectiveneighbouring pixels 101 b located on a lower row of the pixel array.

Referring to FIG. 12b , the memory/storage means 107 of each pixel is aset of Flip-Flops 1201, where the number of Flip-Flops per pixel equalsthe number of bits of the counter. The storage means also contains a setof multiplexers 1202, where the number of multiplexer per pixel is equalto the number of Flip-Flops per pixel. By means of the multiplexers1202, the inputs 1201 b of the Flip-Flops 1201 of a pixel can beselected to be either the output of the counter 108 or the output 1201 cof the Flip-Flops of the pixel on the bottom. The multiplexers 1202 arecontrolled by a global control signal 1206.

The memory/storage means 107 also comprises a pulse generator 1204, thatgenerates a voltage pulse when the output 104 b of the integrator 104indicates that the integral of the difference between the constantcurrent output by the current source 103 and the current output by thephotosensor 102 is equal to, or exceeded the predefined threshold. Thestorage means further comprises a multiplexer 1203. This multiplexer1203 allows to either connect the output of the pulse generator 1204 tothe clock input 1201 a of the Flip-Flops 1201, or a global clock signal1205. The multiplexer 1203 is controlled by the global control signal1206.

During the integration phase, the set of multiplexers 1202 areconfigured so that the output of the counter 108 is connected to theinput 1201 b of the Flip-Flops 1201, and the multiplexer 1203 isconfigured so that the output of the pulse generator 1204 is connectedto the clock input 1201 a of the Flip-Flops 1201.

During the readout phase, the set of multiplexers 1202 are configured sothat the output 1201 c of the Flip-Flops of the neighbouring pixel isconnected to the input 1201 b of the Flip-Flops 1201, and themultiplexer 1203 is configured so that the global clock signal 1205 isconnected to the clock input of the Flip-Flops 1201.

When the readout phase starts, the global clock signal 1205 is pulsedonce (where a pulse means that the clock signal is initially logic low,the it is logic high for some time, and then it is logic low again).This copies the content of the Flip-Flops 1201 in the topmost row ofpixels 1102 a (i.e. the counter values which are stored in thememory/storage means 107 of the pixels in the top row 1102 a of thepixel array 100) are to the readout circuit 1100, and copies the contentof the Flip-Flops of the pixels in a row 1102 b-d to the pixels in therow 1102 a-c above it (e.g. copies the content of the Flip-Flops of thepixels in the second row 1102 b to the pixels in the top row 1102 a, andcopies the content of the Flip-Flops of the pixels in the third row 1102c to the pixels in the second row 1102 b, and copies the content of theFlip-Flops of the pixels in the fourth row 1102 d to the pixels in thethird row 1102 c). When the vision sensor 1200 is used in a depth sensorsuch as the depth sensor 200 shown in FIG. 2a , then the counter valueswhich have been passed from the memory/storage means 107 of therespective pixels in the top row 1102 a to the read out circuit 1100 aresent from the read out circuit 1100 to the processor 204. Afterwards theclock signal is pulsed again. This again copies the content of theFlip-Flops 1201 in the topmost row of pixels 1102 a to the readoutcircuit 1100, and copies the content of the Flip-Flops of the pixels ina row 1102 b-d to the pixels in the row 1102 a-c above it. And so on. Inthis way the content of the Flip-Flops i.e. the counter values which arestored in the memory/storage means 107 of the pixels in the respectiverows 1102 a-d of the pixel array 100 are moved along the rows of pixelsto the read out circuit 1100; and after they reach the read out circuitthe read out circuit 1100 passes the counter values to a processor. Theabove steps are repeated until the counter values which were originallystored in the memory/storage means 107 of the pixels in the lower mostrow 1102 d of the pixel array 100 have been sent to the read out circuit1100 via the pixels in the upper most row 1102 a.

The Flip-Flops 1201 may additionally comprise a reset input that can beused to set the content of the Flip-Flops to a known value, for exampleat the start of the integration phase.

FIG. 13 illustrates a vision sensor 1310 according to a furtherembodiment of the present invention. The vision sensor 1310 in FIG. 13has many of the same features as the vision sensor 1200 in FIG. 12 andlike features are awarded the same reference numbers.

Referring to FIG. 13a , the vision sensor 1310 comprises a row selectioncircuit 1300 having an plurality of row select lines 1301 a-d each linebeing connected to all of the pixels in a respective row 1102 a-d of thepixel array 100. The row select circuit can selectively provide a signalalong a row select line 1301 a-d which causes all of the pixels of thatrow to be connected to the readout circuit 1100 so that the countervalues stored in the storage means/memory 107 of that pixel can bepassed to the read out circuit 1100. Specifically each pixel has aswitch 1308 which can selectively be closed to connect that pixel to theread out circuit; each row select lines 1301 a-d operates the switchesbelonging to a the pixels of a respective row 1102 a-d; when the rowselect circuit passes a signal along a particular row select line 1301a-d then the signal will cause all of the switches of all of the pixelsin that row to close thus connecting each pixel of that row to the readout circuit 1100, so that all of the counter values stored in therespective storage means/memories 107 of the pixels in that row arepassed to the read out circuit 1100. At any given instant in time, atmost one of the row selection signal 1301 is active i.e. only one row ofpixels 1102 a-d of pixels are connected to the read out circuit 1100 atany one time.

The row selection circuit 1300 comprises a shift register comprising aplurality of Flip-Flops 1302; wherein the number of Flip-Flopscorresponds to the number of rows in the pixel array 100. Each Flip-Flop1302 comprises an input 1302 a and an output 1302 b. The output 1302 bof a Flip-Flop 1302 is connected to a respective row select line 1301a-d.

The input 1302 a of a first Flip-Flop 1302 i is connected to a controlsignal line 1304 a (a control signal can be passed from along thecontrol signal line 1304 a to be received at the input 1302 a of a firstflip flop 1302 i); the output 1302 b of a first Flip-Flop 1302 i isconnected to the a first row select line 1301 a; the output 1302 a of afirst Flip-Flop 1302 i is also connected to the input 1302 a of a secondflip flop 1302 ii. The output 1302 a of the second Flip-Flop 1302 ii isconnected to a second row select line 1301 b and it also connected tothe input 1302 a of a third flip flop 1302 iii. The output 1302 a of thethird Flip-Flop 1302 iii is connected to a third row select line 1301 cand it also connected to the input 1302 a of a fourth flip flop 1302 iv.The output 1302 a of the fourth flip-Flop 1302 iv is connected to afourth row select line 1301 d.

A signal output by the first Flip-Flop 1302 i defines a first row selectsignal, and this first row select signal is passed from the output 1302a of the first Flip-Flop 1302 i along the first row select line 1301 a;the first row select signal closes all of the switches of all of thepixels in the first row 1102 a thus connecting each pixel of that row tothe read out circuit 1100, so that all of the counter values stored inthe respective storage means/memories 107 of the pixels in the first row1102 a are passed to the read out circuit 1100.

A signal output by the second Flip-Flop 1302 ii defines a second rowselect signal, and this second row select signal is passed from theoutput 1302 a of the second Flip-Flop 1302 ii along the second rowselect line 1301 b; the second row select signal closes all of theswitches of all of the pixels in the second row 1102 b thus connectingeach pixel of that row to the read out circuit 1100, so that all of thecounter values stored in the respective storage means/memories 107 ofthe pixels in the second row 1102 b are passed to the read out circuit1100.

A signal output by the third Flip-Flop 1302 iii defines a third rowselect signal, and this third row select signal is passed from theoutput 1302 a of the third Flip-Flop 1302 iii along the third row selectline 1301 c; the third row select signal closes all of the switches ofall of the pixels in the third row 1102 c thus connecting each pixel ofthat row to the read out circuit 1100, so that all of the counter valuesstored in the respective storage means/memories 107 of the pixels in thethird row 1102 c are passed to the read out circuit 1100.

A signal output by the fourth Flip-Flop 1302 iv defines a fourth rowselect signal, and this fourth row select signal is passed from theoutput 1302 a of the fourth Flip-Flop 1302 iv along the fourth rowselect line 1301 d; the fourth row select signal closes all of theswitches of all of the pixels in the fourth row 1102 d thus connectingeach pixel of that row to the read out circuit 1100, so that all of thecounter values stored in the respective storage means/memories 107 ofthe pixels in the fourth row 1102 d are passed to the read out circuit1100.

Each of the flip flops 1302 i-iv have their clock input 1302 c connectedto a row clock line 1303 a which is connected to a clock 1303; the clock1303 can generate a clock signal (referred to hereafter as a row clocksignal) which is sent along the row clock line 1303 a to the clockinputs 1302 c of the flip flops 1302 i-iv. The first Flip-Flop 1302 ihas its input 1302 a connected to a control signal line 1304 a which isconnected to a controller 1304. In case the vision sensor is used in adepth sensor such as shown in FIG. 2, the controller may be theprocessor. The controller can generate a control signal which is sentalong control line 1304 a to the input 1302 a of the flip flops 1302i-iv. The processor 204 may define the controller 1304; in other wordsthe processor 204 may be configured to generate a control signal whichis sent along control line 1304 a to the input 1302 a of the flip flops1302 i-iv

It is assumed that before the readout phase, all Flip-Flops 1302 i-ivhave a logic low stored in them. At the start of the readout phase, thecontroller 1304 sends a logic high control signal along the signal line1304 a to the input of the first Flip-Flop 1302 i, and afterwards, theclock 1303 is pulsed once to provide a clock pulse signal (i.e. theclock pulse signal is a sequence consisting of logic low, then logichigh for some time, and then back to logic low), which sends a pulsealong the row clock line 1303 a to all the clock inputs 1302 c of allthe Flip-Flops 1302 i-iv. Now the first Flip-Flop 1302 i will have alogic high stored in it, while all the other Flip-Flops 1302 ii-iv willhave a logic low stored in them. The logic high stored in the firstFlip-Flop 1302 i means that the output 1302 b of the first Flip-Flop1302 i is logic high and the first row select signal 1301 a is active.This first row select signal is passed from the output 1302 a of thefirst Flip-Flop 1302 i along the first row select line 1301 a; the firstrow select signal closes all of the switches of all of the pixels in thefirst row 1102 a and the counter values which are stored in therespective memories/storage means 107 of all of the pixels in the firstrow 1102 a are passed to the read out circuit 1100. The read out circuit1100 will then read out the respective counter values to a processor(such as the processor 204 in FIG. 2a ).

When the readout circuit 1100 has finished reading out the memorycontent of the pixels in the first row 1102 a, the controller 1304 willsend a logic low control signal, along control line 1304 a to the inputof the first Flip-Flop 1302 i, and the clock 1303 sends a clock signal,in the form of a single pulse (which comprises a sequence consisting oflogic low, then logic high and then again logic low), along the rowclock line 1303 a. Now the second Flip-Flop 1302 ii will have a logichigh stored in it, while all the other Flip-Flops 1302 i,iii,iv willhave a logic low stored in them. The logic high stored in the secondFlip-Flop 1302 ii means that the output 1302 b of the second Flip-Flopis logic high and the second row select signal 1301 b is active. Thissecond row select signal is passed from the output 1302 b of the secondFlip-Flop 1302 ii along the second row select line 1301 b; the secondrow select signal closes all of the switches of all of the pixels in thesecond row 1102 b and the counter values which are stored in therespective memories/storage means 107 of all of the pixels in the secondrow 1102 b are passed to the read out circuit 1100. The read out circuit1100 will then read out the respective counter values to a processor(such as the processor 204 in FIG. 2a ).

When the readout circuit 1100 has finished reading the memory content ofthe pixels in the second row 1102 b, the controller 1304 will send logiclow control signal, along control line 1304 a, and the clock 1303 sendsa clock signal, in the form of a single pulse (which comprises asequence consisting of logic low, then logic high and then again logiclow), along the row clock line 1303 a. Now the third Flip-Flop 1302 iiiwill have a logic high stored in it, while all the other Flip-Flops 1302i,ii,iv will have a logic low stored in them. The logic high stored inthe third Flip-Flop means that the output 1302 b of the third Flip-Flopis logic high and the third row select signal 1301 c is active. Thisthird row select signal is passed from the output 1302 a of the thirdFlip-Flop 1302 iii along the third row select line 1301 c; the third rowselect signal closes all of the switches of all of the pixels in thethird row 1102 c and the counter values which are stored in therespective memories/storage means 107 of all of the pixels in the thirdrow 1102 c are passed to the read out circuit 1100. The read out circuit1100 will then read out the respective counter values to a processor(such as the processor 204 in FIG. 2a ).

When the readout circuit 1100 has finished reading the memory content ofthe pixels in the third row 1102 c, the controller 1304 will send alogic low control signal, along control line 1304 a, and the clock 1303sends a clock signal, in the form of a single pulse (which comprises asequence consisting of logic low, then logic high and then again logiclow), along the row clock line 1303 a. Now the fourth Flip-Flop 1302 ivwill have a logic high stored in it, while all the other Flip-Flops willhave a logic low stored in them. The logic high stored in the fourthFlip-Flop means that the output 1302 b of the fourth Flip-Flop is logichigh and the fourth row select signal 1301 c is active. This fourth rowselect signal is passed from the output 1302 a of the fourth Flip-Flop1302 iv along the fourth row select line 1301 c; the fourth row selectsignal closes all of the switches of all of the pixels in the fourth row1102 d and the counter values which are stored in the respectivememories/storage means 107 of all of the pixels in the fourth row 1102 dare passed to the read out circuit 1100. The read out circuit 1100 willthen read out the respective counter values to a processor (such as theprocessor 204 in FIG. 2a ).

When the readout circuit 1100 has finished reading the memory content ofthe pixels in the fourth row 1102 d, controller 1304 will send a logiclow control signal, along control line 1304 a, and the clock 1303 sendsa clock signal, in the form of a single pulse, along the row clock line1303 a. The readout phase has finished, and all Flip-Flops 1302 i-ivhave again a logic low stored in them.

Referring to FIG. 13b there is illustrated the pixel storage/memory 107,which comprises a set of Flip-Flops 1306, where the number of Flip-Flopsper pixel is equal to the number of bits in the counter 108; and a pulsegenerator circuit 1307 that provides a voltage pulse at an output 1307 bof the pulse generator circuit 1307 when integral of difference betweenconstant current output by the photosensor 102 and current output by thecurrent source 103 reaches the predefined threshold (as indicated by theintegrator 104). The output 1307 b of the pulse generator circuit 1307is connected to the clock input 1306 a of the Flip-Flops 1306. The input1306 b of the Flip-Flops is connected to the counter 108. The outputs1306 c of the Flip-Flops are selectively connected to the readoutcircuit 1100 by a set of switches 1308 controlled by the row selectioncircuit 1300.

The Flip-Flops 1306 may additionally comprise a reset input that can beused to set the content of the Flip-Flops to a known value, for exampleat the start of the integration phase.

The readout circuit may take any suitable confirmation. FIG. 14 showsone exemplary implementation for the readout circuit.

Referring to FIG. 14 the readout circuit 1100 comprises one columncircuit 1400 for each column of pixels 101 in the pixel array 100.

Each column circuit 1400 comprises a set of Flip-Flops 1401, where thenumber of Flip-Flops per column-circuit is equal to the number of bitsin the counter 108 and a set of multiplexers 1402 that allows connectingthe input 1401 a of the Flip-Flops of a column circuit either to theoutput 1401 b of Flip-Flops of the previous column circuit or to thestorage means of a pixel 101 in the corresponding column. Themultiplexers 1402 are controlled by a control signal 1404. The clockinputs 1401 c of the Flip-Flops are connected to a column clock signal1403. The output of the last (rightmost in FIG. 14) column circuit formsthe output 1405 of the readout circuit.

If the vision sensor is used in a depth sensor such as in FIG. 2a , theoutput 1405 of the readout circuit is connected to the processor 204.

The vision sensor embodiment shown in FIGS. 12, 13 and 14 areillustrated as comprising four rows and four columns of pixels; however,it should be understood that the pixel array may have any suitablenumber of rows and columns of pixels.

FIG. 15a illustrates a vision sensor 1500 according to a furtherembodiment of the present invention. The vision sensor 1500 in FIG. 15has many of the same features as the vision sensor 1011 in FIG. 11 andlike features are awarded the same reference numbers. FIG. 15billustrate a pixel 101 used in the vision sensor 1500; it will beunderstood that all of the pixels 101 in the pixel array of the visionsensor 1500 will be configured as is illustrated in FIG. 15 b.

Referring to FIG. 15b , each pixel 101 in the vision sensor 1500 shownin FIG. 15a contains two memories 107 a and 107 b, a demultiplexer 1501and a multiplexer 1502. The demultiplexer 1501 allows connecting theoutput of the integrator 104 either to the first memory 107 a or to thesecond memory 107 b. The multiplexer 1502 allows to selectively connecteither the first memory 107 a or the second memory 107 b to the readoutcircuit 1100. The demultiplexer and the multiplexer are controlled by acontrol signal 1503. The pixel shown in FIG. 15b may allow the readoutphase of a frame to happen during the integration phase of the nextframe. One memory instance (for example 107 a) is connected tointegrator and ready to store the output of the counter 108, and theother instance of the memory (in this example 107 b) is connected to thereadout circuit. After the memories 107 b of all pixels are read out andthe integration phase is finished, the connections of the memories areinversed; meaning the memory that was previously connected to theintegrator is now connected to the readout circuit, while the memorythat was connected to the readout circuit is now connected to theintegrator.

If even more memory instances are added to each pixel, the sensor maygather data for multiple frames before reading out the counter valuedata stored in the memories.

FIG. 16a illustrates a vision sensor 1600 according to a furtherembodiment of the present invention. The vision sensor 1600 in FIG. 16has many of the same features as the vision sensor in FIG. 11 and likefeatures are awarded the same reference numbers. FIG. 16b illustrates apixel 101 used in the vision sensor 1600; it will be understood that allof the pixels 101 in the pixel array of the vision sensor 1600 will beconfigured as is illustrated in FIG. 16 b.

Referring to FIG. 16b each pixel contains multiple pixel storagememories (the pixel 101 shown in FIG. 16b comprises three memories 107a, 107 b, 107 c, but it should be understood that any suitable number ofmemories can be used) to allow recording the time instances of multiplethreshold crossings per frame. Each pixel also contains a demultiplexer1601 that connects one of the instances of the memory to the integrator.The demultiplexer is configured such that if the integrator output 104 bsignals that the integral of the difference between the constant currentoutput by the current source 103 and the current output by thephotosensor 102 is equal to, or exceeds, the predefined threshold, acounter value is stored in one of the memories, and afterwards thedemultiplexer automatically connects the next instance of the memoriesto the integrator. Each pixel also contains a multiplexer 1602 thatallows to selectively connect one instance of the memories to thereadout circuit 1100. The multiplexer is controlled by a control signal1603.

Each time the integration of the difference between the current outputby the photosensor 102 and the current source 103 crosses the predefinedthreshold, the output of the counter 108 is stored in one of the memoryinstances, the pixel resets itself (new sampling of backgroundillumination by configuring the current source 103 to output a constantcurrent equal to the current output by the photosensor 102 at thisinstant) and another instance of the memory is connected to theintegrator by means of the demultiplexer. Then the integrator againintegrates the difference between the output of the photosensor and thecurrent source, and if the predefined threshold is crossed again, thecounter value is stored in the instance of the memory that is nowconnected to the integrator.

Various modifications and variations to the described embodiments of theinvention will be apparent to those skilled in the art without departingfrom the scope of the invention as defined in the appended claims.Although the invention has been described in connection with specificpreferred embodiments, it should be understood that the invention asclaimed should not be unduly limited to such specific embodiment.

1. A vision sensor comprising, an array of pixels comprising rows andcolumns of pixels, wherein each pixel in the array comprises, aphotosensor which is configured to output a current proportional to theintensity of light which is incident on the photosensor; a currentsource which is configured such that it can output a current which has aconstant current level which is equal to the current level of thecurrent output by the photosensor at a selected first instant in time,and can maintain that constant current level even if the level of thecurrent output from the photosensor changes after said selected firstinstant in time; an integrator which is configured to integrate thedifference between the level of current output by the current source andthe level of current output by the photosensor, after the selected firstinstant in time; wherein the vision sensor further comprises a counterwhich can measure time, wherein the measure of time is represented by acounter value, wherein the counter is configured such that it can beginmeasure time at the selected first instant; and wherein each pixel inthe array further comprises a storage means which can store the countervalue on the counter at a second instant in time, the second instant intime being the instant when the integration of the difference betweenthe level of current output by the current source and the level ofcurrent output by the photosensor of that pixel reaches a predefinedthreshold level.
 2. A vision sensor according to claim 1 furthercomprising a processor which is configured such that it can receivecounter values stored in the respective storage means each pixel in thearray, and is configured to use the received counter values to generatea depth map.
 3. A vision sensor according to claim 2 further comprisinga read-out circuit for outputting the counter values stored in therespective storage means of one or more pixels in the array to theprocessor.
 4. A vision sensor according to claim 3, wherein the storagemeans of pixels in the same column in the array are connected such thatcounter values stored in the storage means of a pixel may be passed tothe storage means of the adjacent pixel, and wherein the read-outcircuit comprises a plurality of column circuits each of which isconfigured to receive a counter value from the storage means of a pixeland to output the received counter values to the processor, wherein acolumn circuit is provided for each column of pixels in the pixel arraysuch that the number of column circuits correspond to the number ofcolumns in the pixel array, and wherein each column circuit is directlyconnected to a single storage means of a single pixel in each respectivecolumn, and wherein the counter values stored in the storage means ofother pixels in said respective columns can be passed consecutively tothe column circuit via the storage means of said single pixel to whichthat column circuit is directly connected.
 5. A vision sensor accordingto claim 3, wherein the read-out circuit comprises a plurality of columncircuits each of which is configured to receive a counter value from thestorage means of a pixel in a respective column in the pixel array, andto output the received counter values to a processor, wherein a columncircuit is provided for each column in the array of pixels such that thenumber of column circuits correspond to the number of columns in thearray of pixels, and wherein for each pixel in each column a switch isprovided between the storage means of the pixel and the correspondingcolumn circuit for that column, such that each column circuit can beselectively, directly, connected to the storage means of any of thepixels in a corresponding column by closing the switch for that pixel;and wherein the read-out circuit further comprises a row-select circuitwhich can select a row of pixels whose stored counter values are to beoutput to the processor, by selectively closing the switches for pixelsalong a row of the array, such that the storage means of each pixelalong said row is directly connected to it respective correspondingcolumn circuit, so that each corresponding column circuit cansimultaneously receive the counter values stored in the storage means ofthe pixels located in a selected row, and subsequently output thereceived counter values to the processor.
 6. A vision sensor accordingto claim 1 wherein the photosensor comprises a photodiode orphototransistor.
 7. A vision sensor according to claim 6 wherein thephotosensor comprises a photodiode and a cascode NMOS transistor,wherein an output of the photodiode is connected to the source of theNMOS transistor, and the drain of the NMOS transistor defines the outputof the photosensor, wherein the photosensor further comprises aninverting amplifier, wherein the gate of the NMOS transistor isconnected to an output of the inverting amplifier and an input of theinverting amplifier is connected to an output of the photodiode.
 8. Avision sensor according to claim 1 wherein the integrator comprises acapacitor which can integrate the difference between the level ofcurrent output by the current source and the level of current output bythe photosensor, after the selected first instant in time.
 9. A visionsensor according to claim 1 wherein said integrator comprises anamplifier which determines if the integral of the difference between thelevel of current output by the current source and the level of currentoutput by the photosensor is equal to or greater than a predefinedthreshold.
 10. A vision sensor according to claim 1 wherein said currentsource of each pixel in the array comprises a PMOS transistor.
 11. Avision sensor according to claim 9, wherein said current source of eachpixel in the array comprises a PMOS transistor and wherein the gate ofthe PMOS transistor is selectively connected to an output of theamplifier by means of a switch which is located between the gate of thePMOS transistor and an output of the amplifier, wherein the switch isclosed at the first instant in time to cause the current source tooutput a current which has a constant current level which is equal tothe current level of the current output by the photosensor, and whereinthe switch is open between said first instant in time and second instantin time.
 12. A method of vision sensing the method comprising the stepsof, for one or more pixels in an array of pixels, (a) samplingbackground illumination the pixel at a first time instant; (b) after thefirst time instant, integrating the difference between the sampledbackground illumination and a current illumination of said pixel; (c)measuring the time between the first time instant and a second timeinstant when the integral of the difference between the sampledbackground illumination and the current illumination of said pixelreaches a predefined threshold level; and (d) storing the timemeasurement in a storage means of the pixel.
 13. A method of visionsensing according to claim 12, further comprising the steps of, for eachof said one or more pixels, outputting the stored time measurement to aprocessor; and at the processor, generating a depth map using the timemeasurements.
 14. A method of vision sensing according to claim 12,wherein the method comprises using a vision sensor to perform the steps(a)-(c), and, wherein the step (a) comprises identifying the currentwhich is output from the photosensor at the first time instant, andconfiguring the current source so that it outputs a current with aconstant current level which is equal to the current level of thecurrent output by the photosensor at the first time instant, andmaintaining that constant current level until the second time instant atleast, even if the level of the current output from the photosensorchanges after the first time instant, wherein step (b) comprisesintegrating the difference between the current output from thephotosensor and the constant current output by the current source.
 15. Adepth sensor assembly comprising, a projector; and a camera whichcomprises the vision sensor according to claim 1.